Stormwater detention systems are used to control water runoff resulting from rainfall. Such detention systems can help reduce occurrences of, for example, downstream flooding, soil erosion and water quality degradation by collecting the rainfall and controllably discharging the collected water from the detention system.
Often times, communities require that land developments include some form of stormwater control system that limits the discharge of stormwater to a certain rate or rates. These required rates may correspond to the rates of stormwater runoff before the property was developed. The allowable rate in a particular community may change depending on the type of storm. For example, some communities may allow higher stormwater discharge rates during more severe storms that include relatively large amounts of rainfall and require lower stormwater discharge rates during less severe storms that include relatively small amounts of rainfall.
Commonly used detention systems provide a large storage volume (e.g., a buried tank or a detention pond) that begins to fill as soon as stormwater runoff begins. The large storage volume has an outlet that is sized to provide a certain output flow rate when the head in the storage volume reaches its maximum. However, when the level of water in the storage volume is low, the output flow rate can in many cases be less than that which is permitted by applicable codes, regulations, etc. It would be desirable to provide a detention system that begins to output stormwater at the permitted rate relatively quickly and/or that does not begin to fill the storage volume until the water inflow rate exceeds the permitted outflow rate.